Your daughter's godmother certainly has very odd expectations of the role any young people these days play in the lives of their elders!
I am very sorry that this discussion with her has cropped up just when you and your daughter are grieving for your husband and father. Like everyone else I am sorry for your loss.
I am 71 and to me the days of elders expecting to be looked after in their old age by their children, nieces, nephews or god-children were over and done with when my grandparents were of retirement age.
This might be a way of broaching the subject to your friend, if you feel up to doing so. I doubt though that you will influence her in any way, so I would save myself the trouble, if I were you.
What I would do, is mention the subject to your daughter - making it very clear how unreasonable her godmother is being in this matter.
I realise that your daughter, like you, is coping with a lot of emotional issues right now, but if there is the slightest risk of her godmother coming out with the spiel she has given you to your daughter, you would do well to warn her.
The risk of your poor daughter being taken unawares and therefore promising something that she neither could nor should be expected to take on would worry me.
Discuss this with her, so she is forewarned.
If the matter is brought up again both you and she need to stand firm and tell the godmother that neither of you can or are able to care for her. If you feel you can manage to do so, you could offer to put her in touch with those who can. Just don't be led into making definite arrangements on her behalf.
If you do so, and they do not suit this woman (which however good they are is almost certain to be the case) she will make it very plain that this is your fault!